linux-kernel - Re: [v5 02/15] mm/huge_memory: add device-private THP support to PMD operations

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Message-ID: <8a2c4803-d613-4592-9114-d68b97bf3fb0@nvidia.com>
Date: Tue, 16 Sep 2025 13:27:51 +1000
From: Balbir Singh <balbirs@...dia.com>
To: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: damon@...ts.linux.dev, dri-devel@...ts.freedesktop.org,
 Andrew Morton <akpm@...ux-foundation.org>, Zi Yan <ziy@...dia.com>,
 Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>,
 Byungchul Park <byungchul@...com>, Gregory Price <gourry@...rry.net>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Alistair Popple <apopple@...dia.com>, Oscar Salvador <osalvador@...e.de>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Ralph Campbell <rcampbell@...dia.com>,
 Mika Penttilä <mpenttil@...hat.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v5 02/15] mm/huge_memory: add device-private THP support to PMD
 operations

On 9/15/25 18:10, David Hildenbrand wrote:
>>>> +
>>>
>>> Wrong indentation.
>>
>> checkpatch.pl did not complain, what I see is
>>
>> +        VM_WARN_ON(!is_pmd_migration_entry(pmd) &&
>> +                !is_pmd_device_private_entry(pmd));
>>
> 
> David complains :)
> 
>> It looks different in your reply (is your email converting tabs to space?
>> did you want me to align the conditions?
> 
> Indeed, looks wrong, not sure why. Likely I messed it up.
> 

Ack

>>
>> +        VM_WARN_ON(!is_pmd_migration_entry(pmd) &&
>> +               !is_pmd_device_private_entry(pmd));
>>
>>
> 
> Exactly.
> 
>>
>>>
>>>> +        if (is_migration_entry(entry) &&
>>>> +            !is_readable_migration_entry(entry)) {
>>>
>>> Dito.
>>>
>>
>> Same as above :)
>>
>>
>>> Wonder if we want to be more explicit.
>>>
>>> if (is_readable_migration_entry(enrty) ||
>>>      is_readable_exclusive_migration_entry)) {
>>>
>>
>> !is_readable_migration_entry => writable entry or read exclusive, did you mean is_writable_migration_entry()
>> above?
> 
> Yes, sorry, my brain was dizzy after all the review lately.
> 
> if (is_writable_migration_entry(enrty) ||
>     is_readable_exclusive_migration_entry))
> 
> [...]
> 

Ack

>>>
>>> Couldn't we do here
>>>
>>> if (!pmd_present(pmdval))
>>>      goto nomap;
>>>
>>> To replace the original pmd_none() .. check.
>>>
>>> A page table must always be present IIRC.
>>>
>>
>> I am not sure about the pmd_none(), a page table may not be present, I've not audited
>> the callers. But I think we can do
> 
> IIRC page tables must always have the present bit set. So we can just simplify to the single pmd_present() check.
> 

Not sure about that one, are you happy if we follow it up later with a separate fix

Balbir